Wood is comprised of two main components--a fibrous carbohydrate, i.e., cellulosic portion, and a non-fibrous component. The non-fibrous portion of the wood is comprised primarily of phenylpropane units, known as lignin. Part of the lignin is between the cellulosic fibers, bonding them into a solid mass, although a substantial portion of the lignin is also distributed within the fibers themselves.
For use in paper-making processes, wood is first reduced to pulp. Pulping of the wood by the well known Kraft or modified Kraft pulping processes results in the formation of a dark colored slurry of cellulose fibers known as "brownstock". The dark color of the brownstock is attributable to the fact that not all of the lignin has been removed during pulping and that lignin that does remain has been chemically modified in pulping to form chromophoric groups. Thus, in order to lighten the color of the brownstock pulp, i.e., to make it suitable for use in printing and writing and for other white paper applications, it is necessary to continue the removal of the remaining lignin by the addition of delignifying materials and by chemically converting any residual lignin into colorless compounds by a process known as "bleaching" or "brightening".
Various bleaching apparatus utilizing chlorine, oxygen or ozone have been tried. In the past, rotary bleachers of various types have been utilized for bleaching low density pulp by a batch operation. U.S. Pat. No. 1,591,070 to Wolf discloses an improvement on such reactors wherein, in one embodiment, the shell includes a plurality of arm members secured to its inner surface thereof and extending radially inwardly. This shell is used in combination with a stationary central shaft which also includes radial arm members in a staggered relation to those of the shell. Thus, when the shell is rotated, the arm members agitate the pulp to break up lumps and cause the fibers to mingle thoroughly with the bleach. In another embodiment, Wolf discloses that the shell may be angled with respect to the horizontal and provided with a feeding device and an outlet so that the pulp can pass through the reactor by gravity or displacement with the continued rotation of the shell.
Later developments included apparatus which are constructed as a tubular or cylindrical reactor with a central shaft having attached arm members. The central shaft is rotated so that the arm members function to distribute the pulp particles within the internal volume of the reactor and thus expose the particles to a gaseous bleaching agent which is introduced into the reactor. With such apparatus, the construction of the shaft and arm members and their required mode of operation are generally complicated. Further, the shaft structure may itself impede proper mixing of the pulp particles with the gaseous bleaching agent as well as interfere with the feeding of the pulp particles through the apparatus.